A cascade heat exchanger as used in a refrigeration system has traditionally been either “shell and tube” or “plate” type in construction. Typical “shell and tube” heat exchangers have a multiplicity of straight tubes which are expanded into opposing tube sheets that are contained within a cylindrical shell. Because of this style of construction, the tubes are held rigidly between the tube sheets and consequently, high axial strain, stresses, and other forces, can occur during relatively large changes in temperature and pressure of the refrigerant which is being utilized. Under these circumstances if the strain and accompanying stresses reaches a high enough value the individual tubes may crack and rupture resulting in cross-contamination of the two refrigerants which are being employed. This may result in damage to the overall refrigeration system.
Shell and tube heat exchangers occupy a relatively large spatial volume for a given heat transfer duty, and is therefore not considered a “compact” heat exchanger. Plate heat exchangers, on the other hand, can be made in a “plate and shell” or a “plate and frame”; or in a “welded plate” configuration. All of the aforementioned forms of the prior art include a stack or multiple of formed plates, having a manifold system which distributes the two refrigerants, being employed, alternatively, between the plates such that one of the refrigerants flows on one side of each of the plates, while the other refrigerant flows on the opposite side of the respective plate. These plate heat exchangers are considered to be “compact,” but because the plates are held rigidly in a given spatial relationship, one relative to the other, high strains and stresses can form in the plate material when the heat exchanger is exposed to large changes in temperature and pressure of the respective refrigerants.
Ultimately, acceptable performance of any form of a prior art cascade heat exchanger depends largely upon uniform flow distribution of both refrigerants within multiple tubes or plates. This uniform flow distribution is typically difficult to achieve with conventional “shell and tube” and “plate” type heat exchangers under two phase flow conditions (that is condensing or evaporating) of the refrigerants.
While the aforementioned prior art cascade heat exchangers have operated with varying degrees of success, problems still remain in their use when deployed in various environments. Chief among the problems exhibited by these prior art devices include the frequent failure of these prior art designs due to the excessively high strain and stress experienced by the tubes and plates as mentioned, above. Still further, these prior art cascade heat exchangers have a very high cost of construction. Moreover, and as mentioned briefly above, these prior art cascade heat exchangers often present a situation where the non-uniform distribution of a two-phase refrigerant flow to multiple circuits or passages within the prior art devices results in relatively poor heat transfer performance. Still further these prior art cascade heat exchangers often have large internal volume and space requirements which is the case for the shell and tube type construction as mentioned, above. Finally, the prior art devices appear to uniformly prevent the reversing of the two refrigerants for purposes of defrosting the prior art device. Therefore, a heat exchanger which avoids the problems associated with the prior art devices utilized, heretofore, is the subject matter of the present invention.